When Aaron Hernandez, the former New England Patriots tight end, was convicted in 2015 of murdering a friend of his, questions arose: Did his years of butting heads on the football field contribute to his violent behavior off the field? And when he killed himself, in April, new questions were asked: Did football play a role in his suicide?

Now that the researchers have determined that Hernandez had a severe form of C.T.E., the degenerative brain disease tied to repeated head hits, such questions are being asked again about the relationship between football and his behavior, only this time by lawyers.

On Thursday, relatives of Hernandez sued the Patriots and the National Football League, accusing them of not doing enough to protect him despite knowing that the game was dangerous and could lead to brain damage.

Reaching a clear resolution in the courts, though, may be difficult.

The N.F.L. has for years fought off challenges by former players who have accused the league of negligence, fraud and a host of other transgressions related to its handling of concussions, and brain injuries more generally.

The N.F.L. has successfully argued that disputes between players and the league are governed by the collective bargaining agreement, not the courts, and that an arbitrator, not a judge, should settle them.

Former players would also have to prove that the head hits they received in the N.F.L. directly led to their troubles off the field, which is a high bar, especially for those, like Hernandez, who also played football in college and in childhood. The league might also argue that Hernandez knew there was a risk he could be injured playing in the N.F.L., particularly because he had played the game for more than a decade before he was drafted.

Even if the league has a high probability of convincing a judge of all these arguments, the Hernandez family’s lawyer, Jose Baez, might be banking on the N.F.L. to settle the case to avoid bad publicity. The league, though, is known to stand its ground because it wants to stop other players from suing as well.

“There’s always a shot in the dark to bring the league to the table,” said Jodi Balsam, a former lawyer at the N.F.L. who teaches at Brooklyn Law School. “These cases come up all the time, and there will be more of them if they settle this without a fight.”

Indeed, in a conference call with reporters on Friday, Joe Lockhart, a spokesman for the league, said that the N.F.L. would “vigorously” fight the suit and that the Hernandez family would “face significant legal issues.”

“Any attempt here to paint Aaron Hernandez as a victim, we believe, is misguided,” he added.

Lockhart also cast doubt on those who are trying to connect football to C.T.E., or chronic traumatic encephalopathy, as well as C.T.E. to violence and suicide. The science, he said, is far from settled.

While researchers have found links between C.T.E. and symptoms like rage, depression and irritability, they cannot definitively say that the disease predicts the behavior. Also complicating the research is that C.T.E. can only be diagnosed posthumously, so scientists rely on a limited number of brains donated for research.

“Nobody can say football and the head hits created Aaron Hernandez” and his behavior, said Brad Sohn, a lawyer who has sued the N.F.L. on behalf of former players. “But the pathology should scare anyone with a rational bone in his body. He had a raisin-like brain of a 70-year-old” even though he was 27.

Still, despite the legal hurdles facing Hernandez’s family, they might still receive money from the N.F.L.

Lockhart said the league had a range of benefits available to the families of deceased players, including survivor’s benefits and annuities.

He also pointed to the class-action settlement that the league reached with retired players in which players who died and were found to have C.T.E. are eligible for up to $4 million.

That benefit, though, is available only to players who had retired and died before the settlement was approved in federal court in 2015. So in effect, the settlement would be a Trojan horse. If the Hernandez family joins the settlement, arguing that he was no longer playing as of 2013 when he was imprisoned, they give up their right to sue the league, Sohn said, without any guarantee of getting money from the settlement.

Balsam, the former N.F.L. lawyer, suggested that Baez, through the suit, is hoping for the remainder of Hernandez’s $40 million contract extension he signed in 2012. After Hernandez was arrested and charged with murder in June 2013, the Patriots released him and refused to pay him a $3 million bonus he was owed.

The Patriots withheld the money because Hernandez’s conduct was “detrimental to the team.” But Hernandez, through a quirk in Massachusetts law, has had his murder conviction overturned because he died before his appeal of the case could be heard. So, in theory, Baez could argue that because Hernandez technically is no longer guilty of murder, the Patriots should repay his family the money he did not receive.

That argument could play out in court over weeks and months, causing the team and the league significant embarrassment. But the league does not seem to mind sticking it out, even in high-profile cases. It has declined to settle a wrongful-death suit by another former Patriot, the Hall of Fame linebacker Junior Seau, who also killed himself and was found to have C.T.E.

“The N.F.L. is making clear on every level that they want to take the General Patton approach as long as possible and make it as expensive and challenging for anyone who tries to fight them in court,” Sohn said. “So it’s a question of, who’s willing to fight these cases and stand up to the bullies.”

A version of this article appears in print on , on Page D1 of the New York edition with the headline: Legal Hurdles Could Hamper Suit Against N.F.L. in Hernandez Case. Order Reprints | Today’s Paper | Subscribe